Iron inhibits the nitric oxide synthesis elicited by asbestos in murine macrophages

The role of nitric oxide in pleural disease

Nitric oxide (NO) regulates various physiological and pathophysiological functions in the lungs. However, there is much less information about the effects of NO in the pleura. The present review aimed to explore the available evidence regarding the role of NO in pleural disease. NO, has a double-edged role in the pleural cavity. It is an essential signaling molecule mediating various physiological cell functions such as lymphatic drainage of the serous cavities, the immune response to intracellular multiplication of pathogens, and downregulation of neutrophil migration, but also induces genocytotoxic and mutagenic effects when present in excess. NO is implicated in the pathogenesis of asbestos-related or exudative pleural disease and mesothelioma. From a clinical point of view, the fraction of exhaled NO has been suggested as a potential non-invasive tool for the diagnosis of benign asbestos-related disorders. Under experimental conditions, NO-mimetics were found to attenuate hypoxia-induced therapy resistance in mesothelioma. Similarly, hybrid agents consisting of an NO donor coupled with a parent anti-inflammatory drug showed an enhancement of the anti-inflammatory activity of anti-inflammatory drugs. However, given the paucity of research work performed over the last years in this area, further research should be undertaken to establish reliable conclusions with respect to the feasibility of determining or targeting the NO signaling pathway for pleural disease diagnosis and therapeutic management.

Iron from a geochemical viewpoint. Understanding toxicity/pathogenicity mechanisms in iron-bearing minerals with a special attention to mineral fibers

Iron and its role as soul of life on Earth is addressed in this review as iron is one of the most abundant elements of our universe, forms the core of our planet and that of telluric (i.e., Earth-like) planets, is a major element of the Earth's crust and is hosted in an endless number of mineral phases, both crystalline and amorphous. To study iron at an atomic level inside the bulk of mineral phases or at its surface, where it is more reactive, both spectroscopy and diffraction experimental methods can be used, taking advantage of nearly the whole spectrum of electromagnetic waves. These methods can be successfully combined to microscopy to simultaneously provide chemical (e.g. iron mapping) and morphological information on mineral particles, and shed light on the interaction of mineral surfaces with organic matter. This review describes the crystal chemistry of iron-bearing minerals of importance for the environment and human health, with special attention to iron in toxic minerals, and the experimental methods used for their study. Special attention is devoted to the Fenton-like chain reaction involving Fe²⁺ in the formation of highly reactive hydroxyl radicals. The final part of this review deals with release and adsorption of iron in biological fluids, coordinative and oxidative state of iron and in vitro reactivity. To disclose the very mechanisms of carcinogenesis induced by iron-bearing toxic mineral particles, crystal chemistry and surface chemistry are fundamental for a multidisciplinary approach which should involve geo-bio-scientists, toxicologists and medical doctors.

The influence of gene-gene and gene-environment interactions on the risk of asbestosis

This study investigated the influence of gene-gene and gene-environment interactions on the risk of developing asbestosis. The study comprised 262 cases with asbestosis and 265 controls with no asbestos-related disease previously studied for MnSOD, ECSOD, CAT, GSTT1, GSTM1, GSTP1, and iNOS polymorphisms. Data on cumulative asbestos and smoking were available for all subjects. To assess gene-gene and gene-environmental interactions, logistic regression was used. The associations between MnSOD Ala −9Val polymorphism and the risk of asbestosis and between iNOS genotypes and asbestosis were modified by CAT –262 C > T polymorphism (P = 0.038; P = 0.031). A strong interaction was found between GSTM1-null polymorphism and smoking (P = 0.007), iNOS (CCTTT)_n polymorphism and smoking (P = 0.054), and between iNOS (CCTTT)_n polymorphism and cumulative asbestos exposure (P = 0.037). The findings of this study suggest that the interactions between different genotypes, genotypes and smoking, and between genotypes and asbestos exposure have an important influence on the development of asbestosis and should be seriously considered in future research on occupational/environmental asbestos-related diseases.

Role of nitric oxide in pathological responses of the lung to exposure to environmental/occupational agents

Conflicting evidence exists as to whether nitric oxide expresses damaging/inflammatory or antioxidant/anti-inflammatory properties. Data presented in this review indicate that in vitro or in vivo exposure to selected environmental or occupational agents, such as asbestos, silica, ozone or lipopolysaccharide, can result in up-regulation of inducible nitric oxide synthase by alveolar macrophages and pulmonary epithelial cells. In the case of silica exposure, evidence consistently supports a damaging/inflammatory role of nitric oxide and/or peroxynitrite in the pathogenesis of lung disease. Although conflicting data have been reported, the majority of published studies suggest that nitric oxide plays a damaging role in pulmonary injury resulting from exposure to ozone or asbestos. In contrast, most information supports an anti-inflammatory role of nitric oxide following exposure to lipopolysaccharide. Further investigation is required to elucidate fully the mechanisms involved in determining the role of nitric oxide in the initiation and progression of various pulmonary diseases.

The role of iron impurities in the toxic effects exerted by short multiwalled carbon nanotubes (MWCNT) in murine alveolar macrophages

Lung toxicity mediated by multiwalled carbon nanotubes (MWCNT) has been widely demonstrated and recently associated with induction of carcinogenic asbestos-like effects, but the chemical features that drive this toxic effect have still not been well elucidated. The presence of metals as trace contaminants during MWCNT preparation, in particular iron (Fe) impurities, plays an important role in determining a different cellular response to MWCNT. Our goal was to clarify the mechanisms underlying MWCNT-induced toxicity with correlation to the presence of Fe impurities by exposing murine alveolar macrophages to two different MWCNT samples, which differed only in the presence or absence of Fe. Data showed that only Fe-rich MWCNT were significantly cytotoxic and genotoxic and induced a potent cellular oxidative stress, while Fe-free MWCNT did not exert any of these adverse effects. These results confirm that Fe content represents an important key constituent in promoting MWCNT-induced toxicity, and this needs to be taken into consideration when planning new, safer preparation routes.

Hematite nanoparticles larger than 90 nm show no sign of toxicity in terms of lactate dehydrogenase release, nitric oxide generation, apoptosis, and comet assay in murine alveolar macrophages and human lung epithelial cells

Three hematite samples were synthesized by precipitation from a FeCl₃ solution under controlled pH and temperature conditions in different morphology and dimensions: (i) microsized (average diameter 1.2 μm); (ii) submicrosized (250 nm); and (iii) nanosized (90 nm). To gain insight into reactions potentially occurring in vivo at the particle-lung interface following dust inhalation, several physicochemical features relevant to pathogenicity were measured (free radical generation in cell-free tests, metal release, and antioxidant depletion), and cellular toxicity assays on human lung epithelial cells (A549) and murine alveolar macrophages (MH-S) were carried out (LDH release, apoptosis detection, DNA damage, and nitric oxide synthesis). The decrease in particles size, from 1.2 μm to 90 nm, only caused a slight increase in structural defects (disorder of the hematite phase and the presence of surface ferrous ions) without enhancing surface reactivity or cellular responses in the concentration range between 20 and 100 μg cm⁻².

Inducible nitric oxide synthase genetic polymorphism and risk of asbestosis

Asbestos, a known occupational pollutant, may upregulate the activity of inducible nitric oxide synthase (iNOS) and thus the production of nitric oxide (NO). This study investigated whether iNOS (CCTTT)_n polymorphism is associated with an increased asbestosis risk in exposed workers. The study cohort consisted of 262 cases with asbestosis and 265 controls with no asbestos-related disease. For each subject the cumulative asbestos exposure data were available. The number of CCTTT repeats was determined following PCR amplification of the iNOS promoter region. Logistic regression was performed to estimate asbestosis risk. The OR of asbestosis was 1.20 (95%  CI = 0.85–1.69) for the LL genotype compared to the combined SL and SS genotypes and 1.26 (95% CI = 0.86–1.85) for the LL genotype compared to the SL genotype. The results of this study are borderline significant and suggest a possible role of iNOS (CCTTT)_n polymorphism in the risk of asbestosis; however, further studies are needed.

Antioxidants prevent the RhoA inhibition evoked by crocidolite asbestos in human mesothelial and mesothelioma cells

Asbestos is a naturally occurring fibrous silicate, whose inhalation is highly related to the risk of developing malignant mesothelioma (MM), and crocidolite is one of its most oncogenic types. The mechanism by which asbestos may cause MM is unclear. We have previously observed that crocidolite in human MM (HMM) cells induces NF-κB activation and stimulates the synthesis of nitric oxide by inhibiting the RhoA signaling pathway. In primary human mesothelial cells (HMCs) and HMM cells exposed to crocidolite asbestos, coincubated or not with antioxidants, we evaluated cytotoxicity and oxidative stress induction (lipid peroxidation) and the effect of asbestos on the RhoA signaling pathway (RhoA GTP binding, Rho kinase activity, RhoA prenylation, hydroxy-3-methylglutharyl-CoA reductase activity). In this paper we show that the reactive oxygen species generated by the incubation of crocidolite with primary HMCs and three HMM cell lines mediate the inhibition of 3-hydroxy-3-methylglutharyl-CoA reductase (HMGCR). The coincubation of HMCs and HMM cells with crocidolite together with antioxidants, such as Tempol, Mn-porphyrin, and the association of superoxide dismutase and catalase, prevented the cytotoxicity and lipoperoxidation caused by crocidolite alone as well as the decrease of HMGCR activity and restored the RhoA/RhoA-dependent kinase activity and the RhoA prenylation. The same effect was observed when the oxidizing agent menadione was administrated to the cells in place of crocidolite. Such a mechanism could at least partly explain the effects exerted by crocidolite fibers in mesothelial cells.

High aspect ratio materials: role of surface chemistry vs. length in the historical "long and short amosite asbestos fibers"

In nanotoxicology the question arises whether high aspect ratio materials should be regarded as potentially pathogenic like asbestos, merely on the base of their biopersistence and length to diameter ratio. A higher pathogenicity of long asbestos fibers is associated to their slower clearance and frustrated phagocytosis. In the past decades, two amosite fibers were prepared and studied to confirm the role of fiber length in asbestos toxicity. Long fiber amosite (LFA) and short fiber amosite (SFA) have here been revisited, to check differences in their surface properties, known to modulate the biological responses elicited. We report: (i) micromorphology (abundance of exposed cylindrical vs. truncated surfaces; (ii) surface reactivity (oxidation and coordination state of surface iron, free radical generation and oxidizing potential); (iii) activation of nitric oxide (NO) synthase in lung epithelial cells, as representative of an inflammatory cell response. LFA shows a higher free radical yield, stimulates, more than SFA, NO production by cells and reacts with ascorbic acid, thus depriving the lung lining layer of its antioxidant defenses. The higher activity of LFA than SFA is ascribed to the presence of Fe2+ ions poorly coordinated to the surface. SFA shows only a large number of loosely bound Fe3+ ions, pristine Fe2+ ions having been oxidized during the grinding process converting LFA into SFA. Several factors determine a higher toxicity of LFA than SFA, beside length. The lesson from asbestos indicates that other features besides aspect ratio contribute to the pathogenic potential of a fiber type. All these aspects should be considered when predicting the possible hazard associated to any new fibrous material proposed to the market, let alone nanofibers.

Nonpulmonary outcomes of asbestos exposure

The adverse pulmonary effects of asbestos are well accepted in scientific circles. However, the extrapulmonary consequences of asbestos exposure are not as clearly defined. In this review the potential for asbestos to produce diseases of the peritoneum, immune, gastrointestinal (GIT), and reproductive systems are explored as evidenced in published, peer-reviewed literature. Several hundred epidemiological, in vivo, and in vitro publications analyzing the extrapulmonary effects of asbestos were used as sources to arrive at the conclusions and to establish areas needing further study. In order to be considered, each study had to monitor extrapulmonary outcomes following exposure to asbestos. The literature supports a strong association between asbestos exposure and peritoneal neoplasms. Correlations between asbestos exposure and immune-related disease are less conclusive; nevertheless, it was concluded from the combined autoimmune studies that there is a possibility for a higher-than-expected risk of systemic autoimmune disease among asbestos-exposed populations. In general, the GIT effects of asbestos exposure appear to be minimal, with the most likely outcome being development of stomach cancer. However, IARC recently concluded the evidence to support asbestos-induced stomach cancer to be "limited." The strongest evidence for reproductive disease due to asbestos is in regard to ovarian cancer. Unfortunately, effects on fertility and the developing fetus are under-studied. The possibility of other asbestos-induced health effects does exist. These include brain-related tumors, blood disorders due to the mutagenic and hemolytic properties of asbestos, and peritoneal fibrosis. It is clear from the literature that the adverse properties of asbestos are not confined to the pulmonary system.

Fluoro-edenite fibers induce expression of Hsp70 and inflammatory response

Many asbestos-like mineral fibers have been detected in the air of mountainous and volcanic areas of Italy and other parts of the world. These fibers have been suspected to be the cause of increased incidences of lung cancer and other lung diseases in these areas. However, the mechanisms of the cellular response and defense following exposure to these microscopic fibers have not been characterized. We continue to study these mechanisms to be able to propose preventive strategies in large populations. The objective of the present study was to determine comparatively biological responses of mesothelial Met-5A and monocyte-macrophage J774 cells following exposure to two types of fluoro-edenite fibers having low and high iron content (labeled 19 and 27, respectively) obtained from Biancavilla (Sicily, Italy). The reference fiber was a non-iron fibrous tremolite from Val di Susa (Piemonte, Italy). The cells were treated with 5, 50, and 100 μg of fibrous matter per 1 ml for 72 hr. We identified several key mechanisms by which cells responded and counteracted the injury induced by these fibers. The fibers caused induction of the heat shock protein 70 (Hsp70), stimulated formation of reactive oxygen species (detected by using DCFH-DA as a fluorescent probe) and NO^• (measured as nitrite). Exposure of cells to the fibers induced lactate dehydrogenase activity and decreased viability. The fluoro-endenite type 27 was the most potent fiber tested, which indicated that iron and possibly manganese contribute significantly to this fiber toxicity. The J774 cells were more sensitive to fluoro-edenite than Met-5A cells suggesting that the primary site of the fiber-induced inflammatory response could be the macrophage rather than the pulmonary epithelium. Fluoro-edenite produces more biological alterations with respect to non-iron tremolite. Hsp70 and free radicals could be important factors in the context of mineral fiber-induced acute lung injury leading possibly to mutagenic effects. We anticipate that pharmacological blockade of the fiber-dependent cellular responses could in long term offer preventive approach to combat lung diseases induced by these fibers.

Malarial pigment haemozoin, IFN-gamma, TNF-alpha, IL-1beta and LPS do not stimulate expression of inducible nitric oxide synthase and production of nitric oxide in immuno-purified human monocytes

BACKGROUND

Enhanced production of nitric oxide (NO) following upmodulation of the inducible isoform of NO synthase (iNOS) by haemozoin (HZ), inflammatory cytokines and LPS may provide protection against Plasmodium falciparum malaria by killing hepatic and blood forms of parasites and inhibiting the cytoadherence of parasitized erythrocytes (RBC) to endothelial cells. Monocytes and macrophages are considered to contribute importantly to protective upregulation of iNOS and production of NO. Data obtained with murine phagocytes fed with human HZ and synthetic HZ (sHZ) indicate that supplemental treatment of those cells with IFN-gamma elicited significant increases in protein and mRNA expression of iNOS and NO production, providing a potential mechanism linking HZ phagocytosis and increased production of NO. Purpose of this study was to analyse the effect of P. falciparum HZ and sHZ supplemental to treatment with IFN-gamma and/or a stimulatory cytokine-LPS mix on iNOS protein and mRNA expression in immuno-purified human monocytes.

METHODS

Adherent immunopurified human monocytes (purity >85%), and murine phagocytic cell lines RAW 264.7, N11 and ANA1 were fed or not with P. falciparum HZ or sHZ and treated or not with IFN-gamma or a stimulatory cytokine-LPS mix. Production of NO was quantified in supernatants, iNOS protein and mRNA expression were measured after immunoprecipitation and Western blotting and quantitative RT-PCT, respectively.

RESULTS

Phagocytosis of HZ/sHZ by human monocytes did not increase iNOS protein and mRNA expression and NO production either after stimulation by IFN-gamma or the cytokine-LPS mix. By contrast, in HZ/sHZ-laden murine macrophages, identical treatment with IFN-gamma and the cytokine-LPS mix elicited significant increases in protein and mRNA expression of iNOS and NOS metabolites production, in agreement with literature data.

CONCLUSION

Results indicate that human monocytes fed or not with HZ/sHZ were constantly unable to express iNOS and generate NOS metabolites even after stimulation with IFN-gamma or a cytokine-LSP mix that were very active on HZ-fed murine phagocytic lines. Present data do not support the hypothesis that monocytes are mediators of anti-parasitic defence in clinical malaria via activation of iNOS and production of NO, and suggest caution in extrapolating data obtained with murine or hybrid systems to human malaria.

Potential toxicity of nonregulated asbestiform minerals: balangeroite from the western Alps. Part 1: Identification and characterization

In the Italian western Alps, asbestos mineralization (both chrysotile and tremolite amphibole) takes place from serpentinites, together with other less common asbestiform minerals not regulated by the current legislation. In the context of a study on the evaluation of the asbestos risk in this area, the possible role played by the associated asbestiform minerals in the overall toxicity of the airborne fraction has been examined. The first mineral investigated was balangeroite [(Mg,Fe2+,Fe3+,Mn2+)42Si16O54(OH)36], an iron-rich asbestiform contaminant of chrysotile from the Balangero mine (Piedmont), which crystallizes as rigid and brittle fibers. In order to prepare a sample in a form appropriate for chemical and cellular tests, the fibers were separated from the rock and comminuted without damage to their crystalline structure and surface state (as confirmed by X-ray diffraction [XRD] and ultraviolet-visible [UV-Vis] spectroscopy). The first properties examined were durability in simulated body fluids (Gamble's solution) and toxicity to epithelial cells. When compared to UICC crocidolite (the amphibole blue asbestos, regarded as the most pathogenic form), balangeroite appears even more durable than crocidolite. Balangeroite and UICC crocidolite showed a similar in vitro cytotoxic effect on a human epithelial cell line, as evidenced by leakage of intracellular lactate dehydrogenase (LDH) activity, which, observed after a 24-h incubation, was dose dependent and maximal at 12 microg/cm2 for each fiber type. Data show that chemical composition, form, durability, and cell toxicity indicate balangeroite as a potentially harmful fibrous mineral that needs to be examined by further chemical and cellular tests.

Potential toxicity of nonregulated asbestiform minerals: balangeroite from the western Alps. Part 3: Depletion of antioxidant defenses

The asbestiform fibrous silicate balangeroite exhibits cytotoxic and oxidative properties similar to those exerted by crocidolite asbestos. In human lung epithelial cells A549, balangeroite, like crocidolite, inhibited the pentose phosphate pathway (PPP), one of the main antioxidant intracellular tools; this inhibition was exerted also when PPP was activated by the redox-cycling compound menadione. PPP inhibition may be accounted for by the inhibition of its rate-limiting enzyme, glucose-6-phosphate dehydrogenase (G6PD). Reduced glutathione (GSH), the most important intracellular antioxidant molecule, was decreased by both balangeroite and crocidolite incubation. This effect was not related to any increased content of oxidized glutathione, or to any enhanced efflux of glutathione, suggesting that balangeroite fibers, like crocidolite, might favor the reaction of GSH with other molecules.

Potential toxicity of nonregulated asbestiform minerals: balangeroite from the western Alps. Part 2: Oxidant activity of the fibers

The asbestiform mineral balangeroite [(Mg,Fe2+,Fe3+,Mn2+)42Si16O54(OH)36], whose toxic potential is unknown, is associated with chrysotile asbestos in the western Alps (Balangero mine, Piedmont, Italy). In order to examine whether such fibers may contribute to the oxidative damage produced by local asbestos dusts when inhaled, balangeroite was studied by means of both cell-free and cellular tests, comparing the results with those concerning the most pathogenic asbestos form, crocidolite. Similarly to the crocidolite surface, iron was mobilized from balangeroite by chelators, to a different extent: deferoxamine > ascorbic acid > ferrozine. Poorly coordinated surface ions, as evaluated from the adsorption of NO as a probe molecule (by both calorimetry and infrared spectroscopy), are even more abundant on balangeroite than on crocidolite. The spin trapping technique shows that surface iron-derived Fenton activity (HO* from H2O2) is similar for the two fiber types, while a pretreatment in ascorbic acid, by reducing previously oxidized surface iron, activates the potential to cleave a C-H bond (yielding *CO2- from formate anion). Balangeroite, like crocidolite, produces nitrite accumulation, lipid peroxidation, and NO synthase activation in a human lung epithelial cell line (A549). All these findings, regarded as features related to the toxic potential of asbestos, suggest that balangeroite may be a potentially hazardous fiber per se and could be partly responsible for lung diseases reported in epidemiological studies in exposed miners.

G972R IRS-1 variant impairs insulin regulation of endothelial nitric oxide synthase in cultured human endothelial cells

BACKGROUND

Impaired insulin-mediated vasodilation might contribute to vascular damage in insulin-resistant states. Little is known about insulin regulation of nitric oxide (NO) synthesis in insulin-resistant cells. The aim of this work was to investigate insulin regulation of NO synthesis in human umbilical vein endothelial cells (HUVECs) carrying the IRS-1 gene G972R variant, known to be associated with impaired insulin activation of the PI3-kinase (PI3-K) pathway in transfected cells.

METHODS AND RESULTS

HUVECs were screened for the presence of the G972R-IRS-1 (HUVEC-G972R) variant by restriction fragment length polymorphisms. After 24-hour exposure to 10(-7) mol/L insulin, endothelial NO synthase (eNOS) mRNA (reverse transcription-polymerase chain reaction), eNOS protein levels (Western blotting), and NOS activity (conversion of [(3)H]arginine into [(3)H]citrulline) were increased in wild-type HUVECs (HUVEC-WT), whereas they did not change from baseline in HUVEC-G972R. Compared with HUVEC-WT, in HUVEC-G972R after 2 and 10 minutes of insulin stimulation, IRS-1-associated PI3-K activity was reduced by 47% and 32%, respectively; Akt phosphorylation was decreased by 40% at both time points; and eNOS-Ser1177 phosphorylation was reduced by 38% and 51%, respectively. In HUVEC-WT, eNOS-Thr495 phosphorylation decreased after insulin stimulation. In contrast, in HUVEC-G972R, eNOS-Thr495 phosphorylation increased after insulin stimulation and was 40% greater than in HUVEC-WT.

CONCLUSIONS

Our data demonstrate that genetic impairment of the (IRS)-1/PI3-K/PDK-1/Akt insulin signaling cascade determines impaired insulin-stimulated NO release and suggest that the G972R-IRS-1 polymorphism, through a direct impairment of Akt/eNOS activation in endothelial cells, may contribute to the genetic predisposition to develop endothelial dysfunction and cardiovascular disease.

Multiple roles of oxidants in the pathogenesis of asbestos-induced diseases

Exposure to asbestos causes cellular damage, leading to asbestosis, bronchogenic carcinoma, and mesothelioma in humans. The pathogenesis of asbestos-related diseases is complicated and still poorly understood. Studies on animal models and cell cultures have indicated that asbestos fibers generate reactive oxygen and nitrogen species (ROS/RNS) and cause oxidation and/or nitrosylation of proteins and DNA. The ionic state of iron and its ability to be mobilized determine the oxidant-inducing potential of pathogenic iron-containing asbestos types. In addition to their capacity to damage macromolecules, oxidants play important roles in the initiation of numerous signal transduction pathways that are linked to apoptosis, inflammation, and proliferation. There is strong evidence supporting the premise that oxidants contribute to asbestos-induced lung injury; thus, strategies for reducing oxidant stress to pulmonary cells may attenuate the deleterious effects of asbestos.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) sequentially upregulates nitric oxide and prostanoid production in primary human endothelial cells

Endothelial cells express tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptors, but the function of TRAIL in endothelial cells is not completely understood. We explored the role of TRAIL in regulation of key intracellular signal pathways in endothelial cells. The addition of TRAIL to primary human endothelial cells increased phosphorylation of endothelial nitric oxide synthase (eNOS), NOS activity, and NO synthesis. Moreover, TRAIL induced cell migration and cytoskeleton reorganization in an NO-dependent manner. TRAIL did not activate the NF-kappaB or COX-2 pathways in endothelial cells. Instead, TRAIL increased prostanoid production (PGE2=PGI2>TXA2), which was preferentially inhibited by the COX-1 inhibitor SC-560. Because NO and prostanoids play a crucial role in the state of blood vessel vasodilatation and angiogenesis, our data suggest that TRAIL might play an important role in endothelial cell function.

Crocidolite asbestos inhibits pentose phosphate oxidative pathway and glucose 6-phosphate dehydrogenase activity in human lung epithelial cells

The cytotoxicity of asbestos has been related to its ability to increase the production of reactive oxygen species (ROS), via the iron-catalyzed reduction of oxygen and/or the activation of NADPH oxidase. The pentose phosphate pathway (PPP) is generally activated by the cell exposure to oxidant molecules. Contrary to our expectations, asbestos (crocidolite) fibers caused a dose- and time-dependent inhibition of PPP and decreased its activation by an oxidative stress in human lung epithelial cells A549. In parallel, the intracellular activity of the PPP rate-limiting enzyme, glucose 6-phosphate dehydrogenase (G6PD), was significantly diminished by crocidolite exposure. This inhibition was selective, as the activity of other PPP and glycolysis enzymes was not modified, and was not attributable to a decreased expression of G6PD. On the opposite, the incubation with glass fibers MMVF10 did not modify PPP and G6PD activity. PPP and G6PD inhibition did not correlate with the increased nitric oxide (NO) production elicited by crocidolite in A549 cells. Experiments with the purified enzyme suggest that crocidolite inhibits G6PD by directly interacting with the protein. We propose here a new mechanism of asbestos-evoked oxidative stress, wherein fibers increase the intracellular ROS levels also by inhibiting the main antioxidant pathway of the cell.